1. Field of the Invention
The present invention relates to a positive-working, radiation-sensitive mixture which contains an organic polymeric binder which is insoluble in water but soluble in aqueous alkaline solution and an IR-absorbing dye or pigment. It also relates to a recording material comprising a substrate and a layer of this mixture and a process for the production of lithographic printing plates from the recording material. The layer has high sensitivity in the IR range so that the recording material is suitable for direct thermal image production by the computer-to-plate CTP method.
2. Description of the Related Art
The use of dyes and pigments as IR absorbers in radiation-sensitive mixtures is generally known in the art. For example, the recording material according to WO 96/20429 comprises a layer with IR-absorbing carbon black pigments, 1,2-naphthoquinone-2-diazidosulfonic esters or -carboxylic esters and a phenol resin. The 1,2-naphthoquinone-2-diazidosulfonic acid or -carboxylic acid can also be directly esterified with the hydroxyl groups of the phenol resin. The layer is first exposed uniformly to UV radiation and then imagewise to IR laser beams. As a result of the action of the IR radiation, specific parts of the layer rendered soluble by the UV radiation become insoluble again. This is therefore a negative-working system. The processing of the material is thus relatively complicated.
EP-A 0 784 233 likewise describes a negative-working mixture which contains a) novolak and/or polyvinylphenol, b) amino compounds for curing the component a), c) a cyanine and/or polymethine dye which absorbs in the near IR range and d) photochemical acid formers.
The non-prior published Patent Application DE 197 39 302 describes a positive-working, IR-sensitive mixture which comprises a binder which is insoluble in water but soluble, at least swellable in aqueous alkali and carbon black particles dispersed in said binder. The carbon black particles are the radiation-sensitive component essential for imagewise differentiation.
WO 97/39894 describes layers which contain dissolution-inhibiting additives. The additives reduce the solubility of the layer in the unexposed parts in aqueous alkaline developers. These additives are, in particular, cationic compounds, especially dyes and cationic IR absorbers, such as quinolinecyanine dyes, benzothiazolecyanine dyes or merocyanines, in addition to various pigments. However, if these layers are heated to 50 to 100.degree. C. for from 5 to 20 s, the additives lose their inhibiting effect, and the layer becomes soluble in aqueous alkaline solutions.
The positive-working mixture disclosed in EP-A 0 823 327 contains cyanine, polymethine, squarylium, croconium, pyrylium or thiopyrylium dyes as IR absorbers. Most of these dyes are cationic and have an inhibiting effect. In addition, many of them are halogen-containing. Under unfavorable conditions, environmentally harmful decomposition products may form therefrom. However, some dyes having a betaine structure and an anionic dye (compound S-9 on page 7) are also disclosed. After drying of the layer, however, this anionic dye, owing to its large number of sulfonate groups, generally causes crystallization or precipitation of components of the layer, which leads to substantially poorer properties of the IR-sensitive layer and also results in a poor appearance of the layer.
The disadvantage of the layer compositions generally known in the art is that the increase in solubility which is achieved by the post-bake is reversible after storage at room temperature. Thus, if a printing plate is not further processed immediately after baking (e.g. using a heating oven), the development properties change. Thus, reproduction problems during the processing of the recording materials may also result. In addition, many cationic additives are halogen-containing, so that environmentally harmful decomposition products may form under unfavorable conditions.